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Future Watershed Drretoproent <br />Total urbanization of the Lonf Lake watenhed is anacipaled within the next two or tfuec <br />decades. Increased runoff volume and nutrient loading axe expected as a result of this <br />change in land use. This in turn will impaa the lake’s water qiality. To maintain the <br />quality of the lake’s water within the normal limits, existing wetland xparsed over the <br />watershed siujuld be preserved. In addition, storm water detention basins have to be <br />introduced. These basins wUI provide adequate storage for storm water and will reduce the <br />nutrients loading that will eventually enter the lake. The size of such detention basins is a <br />function of the nutrients (mainly phosphorus) loading and the runoff, which in turn is a <br />function of the drainage area (size and use.) <br />For a given subwatershed, the runoff volume in acre-ft, R, cani be estimated using the <br />rational method: <br />R CiA <br />where.C * Runoff Coefficient <br />i ■■ Total Rainfall (ft) <br />A I* Drainage Area (acres) <br />The runoff coefficient, C, will change as ?. result of the change in the subwatershed land use <br />(development). Therefore, the runoff volume will also change. The detention basin should <br />be large enough so that it could store this increased volume of runoff. <br />Considering 0.5 feet of total rainfall (100-ycar, 24-hour duration), the volume of the basm m <br />acre-tt, V required to detain the increased volume of runoff, can be calculated as follows.